This invention relates to the field of integrated circuit processing. More particularly the invention relates to a system for removing chlorine from process materials.
The method by which integrated circuits are manufactured is, in its simplest form, a series of general process steps that are repeated over and over again until the several layers that comprise an integrated circuit have been formed. These steps generally include depositing a layer of material, applying photoresist on top of the deposited layer, patterning the photoresist, etching the deposited layer in those areas where the patterned photoresist has been removed, and then removing the photoresist to expose the patterned deposited layer.
Many of the etchants used to etch the deposited layer contain chlorine. Chlorine is a preferred constituent of etchants because chlorine tends to be a relatively reactive material. Chlorine tends to exist in an ionized state in aqueous solutions such as aqueous etchant solutions. Further, gaseous chlorine compounds tend to dissociate easily, which again produces relatively reactive chlorine ions. The reactive chlorine ions rigorously attack the deposited material in those areas where the patterned photoresist has been removed. This results in an etching process that proceeds at a relatively fast rate.
Unfortunately, even though the integrated circuits are typically rinsed with water following an etching process, this mechanical rinsing action tends to leave some of the chlorine from the etchant on the exposed surfaces of the integrated circuit. Because the chlorine tends to be a reactive material, as described above, the residual chlorine tends to cause problems with the integrated circuit. For example, the chlorine can continue to etch the materials of the integrated circuit. Even though such continued etching may proceed at a very slow rate due to the small amount of chlorine that is left behind, this situation reduces the reliability and utile life of the integrated circuit.
By further example, the residual chlorine, especially in the presence of water, whether that water be from humidity in the air or an aqueous solution, can enable galvanic reactions and other corrosive reactions in and between metallic layers on the integrated circuit. For example, in an aluminum layer, chlorine can react with the aluminum according to the following reactions:
Al+3Cl=AlCl3 2AlCl3+3H2O=Al2O3+6HCl Al+HCl=AlCl3
The reactions according to the above equations tend to proceed until various conditions, such as material availability, reaction kinetics, and thermodynamics, inhibit them. Thus, according to these equations, a deposited aluminum layer, such as a conduction layer, is corroded to aluminum oxide and other byproducts.
What is needed, therefore, is a system for more fully removing or otherwise neutralizing the residual chlorine that may exist on an integrated circuit, without otherwise chemically or mechanically impairing the functional layers of the integrated circuit.
The above and other needs are met by a method for etching a surface of an integrated circuit. A layer of photoresist is applied to the surface of the integrated circuit. The layer of photoresist is exposed and developed, and the surface of the integrated circuit is etched with an etchant that contains chlorine. The surface of the integrated circuit is exposed to tetra methyl ammonium hydroxide to neutralize the chlorine, and rinsed with water.
The benefit of a method according to the present invention is that the tetra methyl ammonium hydroxide does not just mechanically remove the chlorine, such as is accomplished by a rinse, but also chemically neutralizes the chlorine, such as according to the following reaction:
HCl+(CH3)4NOH=H2O+(CH3)4NCl
Without being bound by theory, reactions such as this tend to preferentially bind the chlorine, thus making it unavailable for the galvanic and other corrosive reactions that are detrimental to the deposited layers of the integrated circuit, such as those reactions given above.
Further, tetra methyl ammonium hydroxide does not unfavorable react with the other components and chemistries of the integrated circuit. Tetra methyl ammonium hydroxide is also used in other aspects of integrated circuit processing, such as in developing photoresist. However, tetra methyl ammonium hydroxide has not previously been used after an etch step to neutralize chlorine and other halogens.
In various preferred embodiments of the invention, the step of exposing the surface of the integrated circuit to tetra methyl ammonium hydroxide is accomplished by rinsing the surface of the integrated circuit with an aqueous solution of tetra methyl ammonium hydroxide. Similarly, the surface of the integrated circuit can be exposed to a gas containing tetra methyl ammonium hydroxide, or a gaseous form of tetra methyl ammonium hydroxide.
The step of exposing the surface of the integrated circuit to the tetra methyl ammonium hydroxide can be accomplished either prior to, concurrently with, or after an additional step of removing the layer of photoresist from the surface of the integrated circuit. The photoresist can be removed by ashing in an oxygen plasma or an ozone plasma.
In one embodiment of the invention, the steps of etching the surface of the integrated circuit, exposing the surface of the integrated circuit to tetra methyl ammonium hydroxide, and removing the layer of photoresist from the surface of the integrated circuit are accomplished in separate chambers of a multichamber tool. In a further related embodiment, the step of rinsing the integrated circuit with water is accomplished in the same chamber as that used to expose the integrated circuit to the tetra methyl ammonium hydroxide.